There has been an increasing interest in the development of low bit rate speech coding technologies that can operate at rates of 2400 bit per second (b/s) and below for both current military use and for future commercial applications. Although Government and industry have begun to pursue new coding methodologies which can yield high quality speech at bit rates in the 2400 b/s range, relatively less resources have been applied to efforts regarding development of a good quality 1200 b/s coding that can either be used as a stand-alone coder or as an embedded coder in a higher rate variable rate coder.
The use of Line Spectral Pair (LSP) or Line Spectral Frequency (LSF) representation for vector quantization of short-term spectral parameters is very well known. For example, U.S. Pat. Nos. 5,012,518 and 4,975,956 disclose techniques for vector quantization of LSP parameters. However, the technique described in these patents requires a significantly higher computational overhead than an alternative encoder employing split vector quantization encoding. For example, in split-vector quantization using 20 bits, a maximum of 2048 comparisons are needed to arrive at the optimal quantized LSF vector. In contrast, the conventional method of vector quantization using 20 bits requires more than a million comparisons to arrive at the optimal quantized LSF vector. LSF's are ideally suited for split vector quantization techniques due to its ordered and localized spectral sensitivity properties.
U.S. Pat. Nos. 5,187,745, 5,179,594, 5,173,941 and 5,086,475 disclose CELP vocoders.